A50 Uttoxeter - Project A Flood Risk Assessment & Drainage ......Drainage Strategy to support planning application for the proposed improvements at the junction between the A50 and

A50 Uttoxeter - Project A Flood Risk Assessment & Drainage Strategy Staffordshire County Council

29 May 2014

A50 Uttoxeter - Project A Flood Risk Assessment & Drainage Strategy

Atkins Flood Risk Assessment & Drainage Strategy | 3.0 | 29 May 2014 | 5122675-300

Notice

This document and its contents have been prepared and are intended solely for Staffordshire County Councils information and use in relation to a planning application for a proposed junction improvement scheme on the A50 and A522 in Uttoxeter, Staffordshire known at A50 Uttoxeter – Project A.

Atkins Ltd. assumes no responsibility to any other party in respect of or arising out of or in connection with this document and/or its contents.

This document has 55 pages including the cover and appendices.

Document history

Job number: 5122675-300 Document ref: 5122675-300-R.001

Revision Purpose description Originated Checked Reviewed Authorised Date

1.0 For client comment NOS PJB CB CB 09/05/14

2.0 Appendices updated. Project name changed.

NOS PJB CB CB 19/05/14

3.0 Floodplain compensation updated. Drainage strategy revised.

NOS PJB CB CB 29/05/14



Table of contents

Chapter Pages

1. Introduction 5

2. Background Information 6 2.1. Environment Agency 6 2.2. Staffordshire County Council 7

3. Site Description 8 3.1. Location 8 3.2. Topography 8 3.3. Ground Conditions 9 3.4. Water Environment 9

4. Proposed Works 11 4.1. Description 11 4.2. Sequential Test 11

5. Flood Risk Assessment 12 5.1. Flooding History 12 5.2. Identification of Flooding Sources 12 5.3. Risk Assessment 13 5.4. Flood Risk Management 16 5.5. Residual Risks 17

6. Surface Water Drainage Strategy 18 6.1. Existing Drainage Regime 18 6.2. Proposed Outfalls 19 6.3. Maximum Permitted Discharge Rate 19 6.4. SuDS Selection 20 6.5. Strategy 21 6.6. Adoption & Maintenance 21

7. Conclusions & Recommendations 22 7.1. Conclusions 22 7.2. Recommendations 22

Appendices 23

Appendix A. General Information 24 A.1. Flood Risk and Flood Probability 24 A.2. National Planning Policy Framework (NPPF) 24

Appendix B. Correspondence 26

Appendix C. Drawings 27

Appendix D. SuDS Selection 28

Appendix E. Surface Water Calculations 30



Tables Table 5-1 Floodplain compensation scheme design results 16 Table 6-1 Existing highway drainage discharge flow rates 18 Table 6–2 Existing greenfield equivalent surface water run-off rates 19 Table 6–3 Proposed highway drainage outfalls 19 Table 6-4 Maximum permitted discharge rates 19 Table 6-5 1 in 100-year +20% required storage volumes 21

Figures Figure 3–1 Location plan (Not to scale) 8 Figure 5–1 Historic flood extents plan 12 Figure 5–2 Environment Agency Flood Map 13 Figure 5–3 1 in 100-year +20% flood extents 14 Figure 5–4 Environment Agency surface water Flood Map 15


Atkins Flood Risk Assessment & Drainage Strategy | 3.0 | 29 May 2014 | 5122675-300 5

1. Introduction

Atkins has been commissioned by Staffordshire County Council to prepare a Flood Risk Assessment and Drainage Strategy to support planning application for the proposed improvements at the junction between the A50 and the A522 in Uttoxeter, Staffordshire, referred to as A50 – Project A.

This Assessment has been prepared in accordance with the National Planning Policy Framework1 and

associated Technical Guidance2 and in consultation with the Environment Agency and Staffordshire County

Council. As the proposed works comprise a highway scheme, the Assessment focuses on the impact of the proposed works on flood risk in the immediate area both in terms of the displacement of flood water and increases in drained surface area, and how these will be mitigated.

This document forms one of the appendices in the Environmental Statement prepared for the scheme.

1 National Planning Policy Framework, March 2012, Department for Communities & Local Government.

2 Technical Guidance to the National Planning Policy Framework, March 2012, Department for Communities

& Local Government.



2. Background Information

2.1. Environment Agency The Environment Agency has been consulted regarding the proposed works. Their response is summarised below and included in full in Appendix B:

The proposed works area includes Flood Zones 1, 2 and 3.

The area has been flooded in the past; however, there has not been any significant flooding in the last 10 years and the Environment Agency is not aware of any instances when the A50 carriageway was flooded.

The maximum discharge rate from the proposed works should, if practically and technically feasible, be limited to greenfield rates. If this is not possible, a discharge rate less than the existing discharge rate for the critical rainfall event should be used. All maximum discharge rates apply up to and including the 1 in 100-year (with climate change) event.

Any new discharges into the River Tean will be subject to a Flood Defence Consent. Any applications must be made to the Environment Agency for both the temporary and permanent works.

New discharges into any Ordinary Watercourse must be discussed and agreed with Staffordshire County Council Flood Risk Management department.

It is recommended that appropriate surface water treatment stages for a main road, as defined by CIRIA C697 The SuDS Manual, are put in place as part of the proposed works.

The Flood Risk Assessment must consider all sources of flood risk relevant to this area. The main source of risk is the River Tean and backwater from the River Dove. The Assessment must consider the critical durations and associated flood extents to establish the fluvial flood risk from both watercourses.

A meeting was held between the Environment Agency and Staffordshire County Council to discuss the proposed scheme at the end of January 2014, and the relevant outcomes from this are summarised below:

The Environment Agency has a model of the River Tean, which is part of the 2011 River Dove Tributaries study; however, this does not extend sufficiently far upstream to reach the location of the proposed works.

The model is an ISIS-TUFLOW model (linked 1D-2D). This will require extending to provide the necessary flood level data for any Flood Risk Assessment. The current model can be made available for this purpose.

Extension of the model will require additional channel cross-sections with a similar spacing to those in the current model. The model should extended upstream to the bridge adjacent to Springfields on the road leading to Stramshall.

It is anticipated that no further hydrological assessments will be required as the current model is sufficiently recent and there have been no changes in procedures since it was originally constructed.

The model of the River Tean was supplied by the Environment Agency; however, it was subsequently discovered that the hydrology used in the model was not appropriate for the proposed use of the model. A method was agreed with the Environment Agency to update the hydrology to suit the model application.



2.2. Staffordshire County Council An enquiry was been sent to Staffordshire County Council Flood Risk Management team regarding the junction improvement scheme. The response received is summarised below and included in full in Appendix B:

As the proposed works involve raising levels in the floodplain of the River Tean, a floodplain compensation scheme will be required.

The Flood Risk Assessment will need to consider the effect the proposed works will have on local Ordinary Watercourses.

Further consultation will be required regarding the highway drainage, in particular the opportunities available to use SuDS.

Land Drainage Consents from Staffordshire County Council will be required for any works affecting Ordinary Watercourses.

There is a history of flooding in the vicinity of The Parks and Torrance Grove to the south of the proposed works.

There may be an opportunity to fit pollution control devices into any new highway drainage systems.

Staffordshire County Council has information relating to the existing highway drainage in the vicinity of the works. Copies of this are included in Appendix B.



3. Site Description

3.1. Location The proposed junction improvement scheme is located on the north-western side of Uttoxeter, Staffordshire. The works area is bounded to the north by the River Tean and its floodplain. To the west is the JCB World Parts Centre. The land to the south-west is part of Park’s Farm and is used for agricultural purposes. To the south-east is a residential area of Uttoxeter. The A50 is a dual carriageway and runs west to east through the works area with the A522 running from south-east to north-west. A location plan is shown in Figure 3–1.

Figure 3–1 Location plan (Not to scale)

3.2. Topography The proximity of the proposed works to the River Tean floodplain means that the natural topography falls gradually from west to east. There are embankments on both sides of the A50 which elevate the A522 to allow it to cross the A50 via an overbridge. The relative alignments of the two roads are such that the embankments run broadly parallel with the A50 for a short distance before the A522 returns to natural ground level. The A522 carriageway level on the overbridge is approximately 98.6m AOD. The A50 at the same location is at a level of approximately 89.0m AOD.

The River Tean floodplain falls from west to east. At the western end of the proposed works area, the floodplain level is approximately 91.6m AOD falling to approximately 86.5m AOD at the eastern end of the works. Adjacent to the existing overbridge, the floodplain level is approximately 87.8m AOD.

The land to the south of the A50 falls gradually in a northerly direction towards the River Tean.

Proposed works boundary (approximate)

River Tean



Drawing number 5122675-300-001 in Appendix C is based on a topographical survey supplied by Staffordshire County Council and illustrates the topography of the proposed works area.

3.3. Ground Conditions To date, no intrusive ground investigations have been undertaken; therefore, the content of this section is based on information available in the public domain from various on-line sources. Information relating to the geology beneath the proposed development site has been obtained from the British Geological Survey GeoIndex

3 and boreholes. The boreholes date from when the A50 in this area was constructed in the early

1980’s.

3.3.1. Soil According to Soilscapes

4, the type of soil within the proposed works is described as loamy soils with naturally

high groundwater. According to information obtained from the British Geological Survey, the soil overlying any superficial deposits are between 0.2m and 0.3m thick.

3.3.2. Superficial Deposits The superficial deposits underlying the works area comprise clay, silt, sands and gravels to the north towards the River Tean, and sands and gravels to the south. Borehole logs indicate that superficial deposits extend to a depth of approximately 5m below ground level. The strata generally alternates between sands and gravels, and clay and silt with each layer being between approximately 1.0m and 1.5m thick.

3.3.3. Bedrock The bedrock found beneath the proposed works area is described as Mudstone. Borehole logs indicate that this is approximately 5.0m below ground level.

3.4. Water Environment

3.4.1. Drainage

3.4.1.1. Public Sewerage

According to information supplied by Staffordshire County Council, there are no public gravity sewers within the proposed works area. The closest gravity sewers are located within the residential area to the south-east.

There is a foul water rising main running along the A522 from adjacent to the JCB World Parts Centre. This crosses the bridge over the A50 and continues in an easterly direction running parallel with the A50. Its size and eventual destination is unknown.

3.4.1.2. Highway Drainage

The A50 is served by a highway drainage system that discharges via a number of outfalls into existing watercourses which drain into the River Tean to the north. The pipe sizes vary between 150mm and 450mm diameter.

There is no information available regarding the highway drainage system serving the A522. The carriageway is drained by gullies; however, the outfall from these is unknown. It is likely that the drains connect into a number of culverted watercourses that flow in a northerly direction into the River Tean. This is subject to confirmation.

3 http://www.bgs.ac.uk/geoindex/

4https://www.landis.org.uk/soilscapes/



3.4.1.3. Land Drainage

There are a number ditches and watercourses north and south of the proposed works area. These primarily have a land drainage function and convey flows to the River Tean. The ditches and watercourses to the south of the A50 pass through culverts beneath it to reach the river. As described above, several of these watercourses and ditches are used as outfalls for the existing highway drainage systems.

There is no evidence of any piped land drainage within the proposed works area; however, it is possible that some drains do exist to control water within the embankments. If any pipework is encountered during construction, it must be diverted and/or reconnected to ensure continued drainage.

3.4.2. Watercourses The River Tean is the largest watercourse in the vicinity of the proposed works. At its closest point, it is located approximately 100m from the A522. This is designated a Main River and flows in an easterly direction towards the River Dove. The Environment Agency is responsible for consenting any works close to or within the channel or floodplain.

There are a number of smaller Ordinary Watercourses that discharge into the River Tean. These all flow through the proposed works area. The flows in these channels are generated by run-off from the agricultural land and residential area to the south of the A50. Staffordshire County Council, as the Lead Local Flood Authority, is responsible for consenting all works within or close to these watercourses.

3.4.3. Waterbodies There are no ponds within the proposed works area. The nearest waterbody is a large pond adjacent to the JCB World Parts Centre. It is assumed that this is a balancing pond for this facility. No records are available regarding its outfall; however, it is likely that flows are discharged into the River Tean.

3.4.4. Groundwater According to the Environment Agency website, the proposed works area is located outside of any Source Protection Zones. The superficial deposits beneath the site are classified as a Secondary A aquifer (permeable layers capable of supporting water supplies at a local rather than strategic scale) and the bedrock is designated a Secondary B aquifer (predominantly lower permeability layers which may store and yield limited amounts of groundwater due to localised features such as fissures, thin permeable horizons and weathering).

No information is currently available regarding groundwater levels beneath the proposed works area.



4. Proposed Works

4.1. Description The proposed works comprise the construction of a new junction between the A50 and A522. The scheme includes the closure of two existing junctions and the construction of a new grade-separated junction with two new roundabouts to the north and south together with new slip-roads onto the A50. The roundabouts and slip-roads are to be constructed on new embankments.

The existing bridge carrying the A522 over the A50 is to be removed and the existing approach embankments re-profiled as part of the floodplain compensation scheme for the proposed works.

Drawing number 5122675-300-002 in Appendix C illustrates the proposed junction improvement scheme. The scheme has been designed by Staffordshire County Council.

4.2. Sequential Test The nature of the proposed works is such that the Sequential Test is not deemed to be applicable in this case.



5. Flood Risk Assessment

5.1. Flooding History The Environment Agency has advised that the proposed works area has been affected by flooding in the past. A historic flood event extents plan has been provided to illustrate the area affected. This is shown in Figure 5–1. It is worth noting that the flood extents encroach across the northern embankment carrying the A522 which could not happen in practice. This is either an error on the map or the flood event occurred before the embankment had been constructed.

Source: Environment Agency

Figure 5–1 Historic flood extents plan

The Environment Agency has advised that there have been no reported flood events in this area for the past 10 years and that there are no records of the A50 ever being flooded.

Staffordshire County Council has advised that flooding has occurred on The Parks and Torrance Grove to the south of the proposed works area. No information is available regarding the cause, source or extents of this flooding.

5.2. Identification of Flooding Sources The NPPF Technical Guidance requires that the risk of flooding from the following sources be considered:

Fluvial (river) – The River Tean is close to and there are a number of ditches and watercourses within the proposed works area hence further consideration of this source of flooding is required.

Coastal, tidal and estuarine – the proposed site is remote from coastal, tidal and estuarine waters hence assessment of this source is not required.

Surface water – the proposed works area is located at the bottom of a hill and adjacent to a dual carriageway which is capable of generating large volumes of surface run-off. There is also a risk that the proposed works could affect existing surface water flow paths and hence the associated flood risk. On this basis, further assessment of this source is deemed necessary.

Groundwater – based on the information available relating to soil conditions, geology and groundwater, the risk of flooding from this source is considered to be low and hence not require further consideration.

Sewers – there are no public sewers within the proposed works area hence the risk of flooding from this source is considered to be negligible.

Artificial sources – there are no impounded water bodies at an elevation higher than the site; therefore, the risk associated with this source is negligible.

On the basis of the above, the risk assessment section below will establish the risks associated with fluvial and surface water flooding.

Proposed works area

River Tean

Historic flood extents



5.3. Risk Assessment

5.3.1. Fluvial (River) The Environment Agency Flood Map, shown in Figure 5–2, shows that the proposed works area includes Flood Zones 1, 2 and 3. The Flood Map does not cover the smaller ditches and watercourses to the south of the A50 as these have catchment areas of less than 3km

2. It should be noted, however, that these will have

associated floodplains that should be taken into consideration by the proposed works.

Source: Environment Agency. © Environment Agency copyright and/or database rights 2014. All rights reserved. © Crown Copyright and database right 2014. Ordnance Survey licence number 100026380.

Figure 5–2 Environment Agency Flood Map

As the proposed works area is shown to contain Flood Zone 3, the flood level must be established to allow the flooding extents to be confirmed using more accurate terrain data than that used to generate the Flood Map.

As previously described in section 2.1, the Environment Agency has a model of the River Tean as part of the wider River Dove Tributaries study undertaken in 2011. This model does not extend sufficiently far upstream on the River Tean to cover the proposed works area; therefore, it has been necessary to extend the model to determine the flood level and consequentially confirm the floodplain extents and the need for compensation.

The existing hydraulic model of the River Tean is an ISIS-TUFLOW (linked 1D-2D) model. This was obtained from the Environment Agency and extended upstream to the bridge adjacent to Springfields on the road leading to Stramshall. This was done using a topographical survey of the channel commissioned as part of this Assessment. The floodplain was represented using LiDAR data.

Proposed works area

River Tean

Flood Zone 3

Flood Zone 2



An initial review of the model in its current state showed that it was not appropriate for generating results for this Assessment. The model was originally used to assess flooding on the River Dove and not on the River Tean and, as such, the model inflows for the River Tean were not suitable as they were steady flow rather than hydrograph inputs. After consultation with the Environment Agency, it was agreed that the model could be made suitable for the intended purpose by changing the River Tean inflow by adjusting the storm area and return period (with a 20% increase in flow to allow for climate change). The downstream extent of the model was also truncated to a point east of the A518 road bridge. As a result of this change, it was also agreed that a sensitivity exercise would be carried out at the downstream boundary to assess the impact of a high magnitude flood event on the River Dove.

A normal depth boundary has been used at the downstream model extent. To check the sensitivity of the downstream boundary to account for a high magnitude flood on the River Dove, the normal depth boundary was replaced with fixed head boundary. The level of the boundary was fixed at the 1 in 100-year plus climate change maximum flood level at the downstream boundary from the existing River Dove model. The assessment indicates that changing the boundary to account for a 1 in 100-year plus climate change flood event on the River Dove has little to no effect on maximum flood levels adjacent to the proposed works area.

The model was run for the 1 in 100 year (plus 20% allowance for climate change) flood event. The resultant flood extents are shown in Figure 5–3 and on drawing number 5122675-300-003 in Appendix C. The flood levels vary from approximately 91.1m AOD at the north-western end of the proposed works to 87.8m AOD to the south-east.

The results show that a smaller amount of the proposed works area lies within the 1 in 100-year +20% floodplain than indicated on the Flood Map shown in Figure 5–2. Where the proposed works encroach into the floodplain, any lost floodplain volume will need to be replaced using a floodplain compensation scheme to ensure that flood water is not displaced and result in flooding elsewhere. This is described in section 5.4.

Figure 5–3 1 in 100-year +20% flood extents

1 in 100-year +20% flood extents

Proposed works area



The watercourse modelling only covered the main River Tean channel; none of the other smaller watercourses and ditches referred to in section 3.4.2 were included. A potential mechanism by which the floodplain extents in the vicinity of the proposed works could be extended is if these smaller channels provided a pathway into land further to the south that could then flood. This is a particular issue where the existing embankments potentially prevent flood water from reaching land further to the south.

A check has been undertaken using the topographical survey to establish whether backwater from the River Tean floodplain could cause flooding on land further to the south. This has shown that, as ground levels gradually rise away from the River Tean, any flood water originating from the River Tean will be wholly contained within the channels and not cause flooding beyond the main floodplain.

The ditches and watercourses from the land south of the A50 may also have associated floodplains. These are expected to be small by virtue of the scale of these channels hence a specific assessment of these has not been undertaken. The proposed works will include measures to ensure that the flows in these watercourses are not restricted and that any floodplains are not changed in a manner that will impact on any nearby properties. Details of these mitigation measures are provided in section 5.4.

5.3.2. Surface Water The Environment Agency surface water flood map, shown in Figure 5–4, shows that the proposed works area contains zones potentially at risk from surface water flooding. These are generally associated with the ditches and watercourses draining the land to the south of the A50, and the channels leading to the River Tean to the north of the A50. The A50 is also indicated to be at risk of surface water flooding. It is worth noting that, according to the surface water flood map, the reported flooding on Torrance Grove and The Parks is likely to be attributable to surface water flooding rather than any other source.

Source: Environment Agency. © Environment Agency copyright and/or database rights 2014. All rights reserved. © Crown Copyright and database right 2014. Ordnance Survey licence number 100026380.

Figure 5–4 Environment Agency surface water Flood Map

Proposed works area

High risk zone

Low risk zone



There is a risk that the proposed works, in particular the construction of embankments, could block or divert some of the existing surface water flow paths which may result in an adverse change in flood risk elsewhere. To mitigate this risk, the proposed scheme will incorporate measures to either safely divert flow paths or allow them to continue unrestricted through the proposed works. Details of these are provided in section 5.4.

5.4. Flood Risk Management The proposed works will incorporate the following flood risk management measures to mitigate the risks identified above:

Any floodplain volume that is lost as a result of the proposed works, in particular the construction of the new embankments, will be replaced using a floodplain compensation scheme. Details of these are provided below.

Existing watercourses and ditches through the proposed works will be culverted and/or diverted as necessary to allow flows to continue unrestricted once the scheme has been completed. Details of these are provided below.

Surface water flow paths through the proposed works area will be diverted as necessary to ensure continuity of drainage. The works to achieve this are anticipated to be combined with those required to maintain the existing watercourses and ditches.

The proposed scheme will include a positive highway drainage system to intercept run-off from the carriageway. Flows from the system will be attenuated to ensure that the discharge rate into the River Tean does not exceed existing rates. Details of the surface water drainage strategy are included in section 6.

5.4.1. Floodplain Compensation The floodplain compensation scheme has been designed in accordance with the current Environment Agency requirement for level-for-level, volume-for-volume replacement of lost floodplain volume. The compensation scheme has been designed using 0.2m thick horizontal slices. The design of the floodplain compensation is based on the flood levels described above and a three-dimensional topographical survey of the proposed works area.

The floodplain compensation scheme was designed by measuring the area of each contour defining the top and bottom of each horizontal slice that was lost as a result of the fill thereby allowing the volume within each slice to be calculated using the trapezium rule. The replacement floodplain was determining by ensuring that the reduction in each contour area is offset elsewhere thereby ensuring that the floodplain volume is fully replaced.

Table 5-1 shows details of the floodplain volume that is lost as a result of the fill together with information relating to the replacement of this volume through cut in specific areas. Drawing number 5122675-300-004 in Appendix C illustrates the areas of proposed cut and fill required by the floodplain compensation scheme.

Table 5-1 Floodplain compensation scheme design results

Level slice (m AOD)

Lost floodplain Replaced floodplain

Contour area lost (m

2) Slice

volume (m3)

Contour area gained (m

2) Slice

volume (m3)

Difference (m

3)

Bottom Top Bottom Top

88.6 – 88.8 39 84 12.3 127 170 29.7 +17.4

88.8 – 89.0 84 115 19.9 170 184 35.4 +15.5

89.0 – 89.2 115 161 27.6 184 200 38.4 +10.8

89.2 – 89.4 161 189 35.0 200 234 43.4 +8.4

89.4 – 89.6 189 52 24.1 234 218 45.2 +21.1

89.6 – 89.8 52 129 18.1 218 174 39.2 +21.1

89.8 – 90.0 129 325 45.4 174 327 50.1 +4.7

Total

182.4

251.7 +99.0



The results show that the floodplain compensation scheme replaces the lost floodplain volume at all levels. For all slices extra floodplain volume is created. This is primarily due to over-excavation at higher levels to enable volume replacement at lower levels to be achieved. In total, the floodplain compensation scheme generates an additional 99.0m

3 of floodplain storage hence the fill required by the proposed scheme will not

adversely affect the performance of the floodplain in the surrounding area.

5.4.2. Maintenance of Flow Paths In order to ensure that flow paths associated with overland flows and the various watercourses and ditches are maintained, these will be diverted and/or culverted through the proposed works area as necessary. Any new diversion channels will be at least the size of the upstream existing channel and increased in size as appropriate to accommodate any additional inflows that may be generated by the proposed scheme. Details of the channels will be confirmed during detailed design.

Similarly, any new culverts will be designed to accommodate the 1 in 100-year flow rate that is anticipated to be generated by the upstream catchment. The size of the culvert will also ensure that sufficient freeboard is provided within the culvert to allow debris to pass through without causing a blockage. The size of the freeboard is subject to discussions with the Environment Agency. The details of the culverts will be confirmed during detailed design.

5.5. Residual Risks Residual risks are the risks that remain once the flood risk management measures described above have been implemented. These are typically associated with extreme events that overwhelm drainage systems and exceed the flood levels used to design any mitigation measures.

The primary residual risks associated with the proposed works are:

An extreme rainfall event which exceeds the capacity of the proposed highway drainage system to both intercept and convey flows. During such an event, water that is unable to enter the formal drainage system will accumulate on and flow along the carriageway towards any low points. This may result in the A50 being flooded.

An extreme rainfall event on the catchment to the south of the A50 may result in flow rates that exceed the capacity of any new culverts and channels that are constructed as part of the proposed scheme. This may result in flooding of land and properties as the water accumulates upstream of any culverts.



6. Surface Water Drainage Strategy

6.1. Existing Drainage Regime The existing carriageway of both the A50 and A522 is positively drained via a system consisting of gullies and pipes. These discharge into a number of small drainage ditches and watercourses that flow in a northerly direction towards the River Tean.

The existing highway drainage system serving the junction in its current form has a number of outfalls into the ditches and watercourses that pass through the proposed works area. As the catchment areas are small and the surface is 100% impermeable, the flow rate discharged to each outfall has been determined using the Modified Rational Method:

Q = 2.78Ai

where Q = run-off flow rate (l/s) A = catchment area (ha) i = rainfall intensity (mm/hr)

A value of 50mm/hr has been used for rainfall intensity. This equates to a 1 in 5-year 15-minute rainfall event which is equivalent to the design standard rainfall event for highway drainage systems. More significant rainfall events will tend to result in standing water on the carriageway due to limitations in the capacity of the collection systems hence the discharge flow rate from the pipework is unlikely to be increased.

Only the system draining the A522 will be altered as a result of the proposed works hence it is important to establish the existing flow rates from these areas so the impact of the junction alterations can be assessed and mitigated. The A50 will continue to be drained in the same manner so it is anticipated that the generated flow rates will remain unchanged. Therefore, the calculations that follow only consider the drainage of the A522.

The outfall locations are illustrated on drawing number 5122675-300-005 in Appendix C. Table 6-1 shows the existing catchment areas draining to each together with the resultant discharge flow rates.

Table 6-1 Existing highway drainage discharge flow rates

Outfall reference Contributing area (ha) Discharge flow rate (l/s)

1 0.802 111.5

3 0.177 24.6

4 0.655 91.0

The proposed works include some construction on land that is currently greenfield. In order to take the run-off rate from these areas into consideration within the surface water drainage strategy, this has been calculated using IH124 Flood Estimation for Small Catchments

5 with modifications as defined in the Interim

Code of Practice for Sustainable Drainage Systems6 as the site has an area of less than 50ha.

MicroDrainage Source Control was used to undertake the calculations using the following parameters:

Area 1ha (in order to obtain a rate per hectare)

SAAR (Standard Annual Average Rainfall) 794mm

SOIL 0.45

Urban 0.0

Region 4

5 IH Report 124 Flood Estimation for Small Catchments, D C W Marshall & A C Bayliss, Institute of

Hydrology, 1994. 6 Interim Code of Practice for Sustainable Drainage Systems, National SUDS Working Group, DEFRA, July

2004.



The calculation results are shown in Table 6–2.

Table 6–2 Existing greenfield equivalent surface water run-off rates

Return period Run-off rate (l/s/ha)

1 in 1-year 4.2

1 in 2-years 4.6

QBAR (1 in 2.33-years) 5.1

1 in 30-years 10.0

1 in 100-years 13.1

6.2. Proposed Outfalls It is anticipated that the highway drainage from the new slip roads and junctions will discharge into the River Tean via existing land drainage ditches in the same manner as the existing system. The ground conditions are not appropriate for infiltration to be used for the disposal of surface water. Wherever possible, existing outfalls will be used; however, it is expected that some new outfalls may need to be created. Table 6–3 shows details of the proposed outfalls and these are illustrated on drawing number 5122675-300-005 in Appendix C.

Table 6–3 Proposed highway drainage outfalls

Outfall reference Location

1 Upstream end of a culvert beneath the A50 located between the A522 and the A50 on the adjacent to the existing overbridge.

2 Upstream end of a culvert beneath the A50 towards the western end of the proposed works area.

3 Existing ditch adjacent to the access into the JCB World Parts Centre off the A522.

4 Existing ditch to the north of the A522 adjacent to the existing junction between the A50 westbound and the A522.

5 Existing ditch adjacent to the lower section of the proposed A50 west-bound on-slip.

6.3. Maximum Permitted Discharge Rate The Environment Agency has advised that the maximum permitted discharge rate from the proposed works should be limited to greenfield equivalent run-off rates; however, where this is not practicable or technically feasible, a discharge rate equivalent to the existing rate is acceptable. On this basis, Table 6-4 shows the discharge rates to each outfall based on the annual average greenfield run-off rate of 5.1l/s/ha as shown in Table 6–2.

Table 6-4 Maximum permitted discharge rates

Outfall reference

Contributing area (ha)

Existing discharge rate (l/s)

Greenfield equivalent rate (l/s)

Maximum permitted discharge rate (l/s)

1 2.018 111.5 10.3 5.0

2 0.205 1.0 1.0 5.0

3 0.861 24.6 4.4 5.0

4 0.614 91.0 3.1 5.0

5 0.162 0.8 0.8 Unrestricted



Outfalls 2, 3 and 4 have a greenfield equivalent run-off rate of less than 5l/s. A flow rate of 5l/s is considered to be the practical minimum in terms of flow controls and reducing the risk of future blockages; therefore, for these outfalls, the maximum permitted discharge rate has been increased to 5l/s. This will ensure that the flow control used is of a size that will not introduce future maintenance issues or be prone to frequent blockage.

Due to land ownership constraints, there is no space to accommodate surface water storage for Outfall 5; therefore, no flow restriction is proposed for this outfall. To offset this, the maximum permitted discharge rate for Outfall 1 has been reduced from 10.3l/s to 5l/s.

These discharge rates apply to all rainfall events up to and including the 1 in 100-year +20% event. As the maximum permitted discharge rates are equivalent to the annual average greenfield run-off rates for each catchment area, this represents a betterment in terms of the flow rates entering the local watercourse network. During the 1 in 100-year +20% rainfall event, the flows entering the local network will be equal to the current average annual flow rates. This reduction in flows for extreme events is considered to adequately offset the effect of increasing some of the flow rates to achieve the minimum of 5l/s and allowing an unrestricted discharge from Outfall 5.

6.4. SuDS Selection The Environment Agency and Staffordshire County Council have requested that SuDS are incorporated into the proposed scheme in accordance with CIRIA C697 The SuDS Manual

7. As the proposed works are a

highway scheme, according to CIRIA C6977, Table 5.2 (reproduced in Appendix D), all SuDS techniques,

with the exception of source control methods, are considered acceptable.

The proposed outfall, the River Tean, is considered to be a medium sensitivity receptor. According to CIRIA C697

7, Table 5.6, 3 treatment train components are required.

The parameters for the initial selection of SuDS components for this site using CIRIA C6977, Table 5.4,

reproduced in Appendix D, are as follows:

Soils: Impermeable Ground conditions are not confirmed. Due to uncertainty of permeable extents and characteristics, the site has been assumed impermeable therefore identify SuDS that will operate regardless of permeability.

Area draining to a single SuDS component: 0 – 2ha

Minimum depth to watertable: >1m

Site slope: 5 – 10%

Available head: 0 – 1m

Available space: High

Based on this initial selection process, the following SuDS techniques are identified as being potentially suitable for this site:

All types of retention

All types of wetland

Bio-retention/filter strips

Filter trenches

All types of open channels/swales

Retention ponds are anticipated to provide the majority of surface water storage for the proposed development. Where space is more restricted, storage within over-sized ditches will be used. Both of these techniques would also provide water treatment. Bio-retention/filter strips and filter trenches are primarily water treatment techniques which can be implemented where the scheme design permits.

Wetlands require a constant through-flow of water and/or consistently high groundwater levels to remain functional. As neither of these is guaranteed to be present on the site, wetlands are not considered viable for this site.

7 C697 The SuDS Manual, CIRIA, 2007



6.5. Strategy The surface water drainage system will primarily comprise a conventional pipe network draining towards attenuation feature before discharging to the outfall. Drawing number 5122675-300-005 in Appendix C illustrates the proposed surface water drainage strategy.

SuDS features as described in section 0 will be incorporated into the system wherever possible. The following outlines a potential approach for implementing SuDS on this development that will achieve the required number of water treatment stages although this is subject to confirmation during detailed design:

Highway run-off intercepted either by trapped gullies or road-side filter drains. Any gullies could also connect into the filter drains.

Ditches and swales will be used in place of pipes where space allows to convey flows and, if necessary, provide additional storage. It is likely that these will be located at the toe of any embankments.

Retention pond to provide primary storage prior to flows being discharged into either the existing highway drainage network or an existing ditch or watercourse.

At this stage, as details of the layout of each drainage networks are not known, it has been assumed that all the surface water storage needed to achieve the maximum permitted discharge rate will be provided by a single attenuation feature immediately upstream of the outfall. The storage volume calculations have been undertaken using MicroDrainage Source Control. The results are summarised in Table 6-5 with full calculations included in Appendix E. The extents of the required storage areas, assuming an active storage depth of 1.0m, are shown on drawing number 5122675-300-005 in Appendix C.

Table 6-5 1 in 100-year +20% required storage volumes

Outfall reference Contributing area (ha)

Maximum permitted discharge rate (l/s)

Storage volume (m

3)

1 2.018 5.0 1,550

2 0.205 5.0 85

3 0.861 5.0 530

4 0.614 5.0 350

5 0.162 Unrestricted 0

6.6. Adoption & Maintenance It is anticipated that all parts of the proposed highway drainage system will be adopted by Staffordshire County Council.



7. Conclusions & Recommendations

7.1. Conclusions The proposed works area contains Flood Zones 1, 2 and 3. The primary sources of flooding affecting the site are fluvial, associated with the River Tean, and surface water from the land to the south of the A50. All other sources of flooding are considered to have a minimal impact on the proposed works area.

Modelling has shown that the proposed works will encroach slightly into the 1 in 100-year +20% floodplain of the River Tean with the loss of some floodplain volume. To mitigate this, a floodplain compensation scheme is proposed to replace this volume. The resultant scheme has been designed in accordance with Environment Agency requirements and provides an additional 99.0m

3 of floodplain storage beyond that

currently present.

The fluvial flood risk associated with a backwater effect up the various small ditches and watercourses that flow through the proposed works area has been determined to be negligible. These ditches and watercourses have associated floodplains; the proposed scheme will ensure that these are maintained and that no restrictions are placed on their flow capacity as they pass through the proposed works area.

The surface water flood risk affecting the proposed works area is associated with the ditches and watercourses draining the land to the south of the A50. These channels will be diverted and culverted through the proposed works area to ensure that flow paths are maintained. Any culverts will be designed to accommodate the 1 in 100-year flow with a freeboard to be agreed with the Environment Agency.

The residual risks once the flood risk management measures have been implemented are associated with extreme rainfall events overwhelming the proposed drainage systems or the ditches and watercourses from the south of the A50. The flood risk management measures are anticipated to reduce the severity of the impact of extreme rainfall events due to the additional capacity that they provide.

The proposed highway drainage system will discharge flows into the existing ditches and watercourses that flow through the proposed works area. The maximum discharge flow rates will be greenfield equivalent rates unless it is demonstrated that this is not practically or technically viable. SuDS can be included within the proposed drainage system and must be configured to provide at least 3 levels of water treatment for all run-off.

Surface water storage is anticipated to be provided by retention ponds and possibly swales/ditches at the toes of the new embankments. The storage is designed to accommodate up to the 1 in 100-year +20% rainfall event.

It is anticipated that all parts of the proposed highway drainage system will be adopted by Staffordshire County Council.

7.2. Recommendations This Flood Risk Assessment is suitable for submitting in support of a planning application for the proposed junction improvement scheme.

The design of the proposed highway drainage systems, new culverts, channel diversions and floodplain compensation must comply with the parameters defined in this document.

Documents

A50 Uttoxeter - Project A Flood Risk Assessment & Drainage ......Drainage Strategy to support planning application for the proposed improvements at the junction between the A50 and